1. Field of the Invention
The present invention generally relates to an order wire device in a communication system, and more particularly to an order wire device having a function for reducing noises caused by ring-back-tones generated from called order wire devices in a communication system, such as an optical communication system.
2. Description of Related Art
In an optical communication network system as shown in FIG. 1, stations ST1, ST2, ST3 and ST4 are coupled to each other by optical fiber cables, and subscriber terminals (not shown) are connected to each station having a switching function. High-speed digital communication is carried out between subscriber terminals via the stations. Order wire devices OW1, OW2, OW3 and OW4 are respectively connected to the stations ST1, ST2, ST3 and ST4. An administrator of each station uses an order wire device for communicating information required for maintenance to an administrator of another station. In calling between order wire devices, it is desired that noise interfering with signals be reduced.
In the optical communication network system as shown in FIG. 1, signals output from each of the order wire devices OW1, OW2 , OW3 and OW4 are superposed, as supplementary signals, on main signals transmitted among the stations ST1, ST2, ST3 and ST4. The communication between the order wire devices is carried out based on the supplementary signals superposed on the main signals.
When an order wire device is called by an order wire device connected to another station, the called order wire device determines whether or not a station number supplied from the calling order wire device is equal to that identifying the called order wire device. When the station number supplied from the calling order wire device is equal to that identifying the called order wire device, the called order wire device outputs a ring-back-tone signal for a predetermined time. An administrator lifts a phone receiver of the called order wire device, so that an off-hook operation of the called order wire device is carried out. Then the called order wire device stops output of the ring-back-tone signal. After this, a communication is established between the calling order wire device and the called order wire device.
There are two modes for calling order wire devices. The first mode is referred to as a single station calling mode. In the single station calling mode, an order wire device can call only one order wire device. The second mode is referred to as an all station calling mode. In the all station calling mode, an order wire device can simultaneously call all order wire devices provided in the communication system.
FIGS. 2A and 2B show the all station calling mode in the communication system having four order wire devices OW1, OW2, OW3 and OW4.
Referring to FIG.2A, a first order wire device OW1 calls other order wire devices OW2, OW3 and OW4 in accordance with the all station calling mode. When the order wire devices OW2, OW3 and OW4 determine that they are being called in accordance with the all station calling mode, the order wire devices OW2, OW3 and OW4 output ring-back-tone (RBT) signals. An RBT signal output from the second order wire device OW2 is transmitted to the first order wire device OW1 via two stations respectively connected to the second and first order wire devices OW2 and OW1. An RBT signal output from the third order wire device OW3 is transmitted to the first order wire device OW1 via three stations respectively connected to the third, second and first order wire devices OW3, OW2 and OW1. An RBT signal output from the fourth order wire device OW4 is transmitted to the first order device OW1 via four stations respectively connected to fourth, third, second and first order wire devices OW4, OW3, OW2 and OW1.
Conventionally, when an off-hook of one of the order wire devices OW2, OW3 and OW4, for example, the second order wire device OW2, is carried out, the order wire devices OW2, OW3 and OW4 assume states shown in FIG. 2B. That is, the second order wire device OW2 stops output of the RBT signal, but the remaining order wire devices OW3 and OW4 continuously output the RBT signals for a predetermined time. In this state, the RBT signals output by the third and fourth order wire devices OW3 and OW4 are superposed, as noise, on sound signals in the call between the first and the second order wire devices OW1 and OW2. Thus, the quality of the sound signals in the call between the first and second order wire devices OW1 and OW2 deteriorates.